Information carriers having a plurality of information layers are widely used. For example, certain DVDs comprise a first and a second information layer which can be scanned by means of a radiation beam intended to be focused on one of the information layers in order to read from or record to the selected information layer. The expression “scanning” means either reading or writing data from or to an information layer.
An optical scanning device for scanning such a dual-layer DVD usually comprises means for generating a diverging radiation beam, a collimator for converting this diverging beam into a parallel radiation beam and an objective lens for focusing said parallel radiation beam on one of the two information layers. The optical scanning device further comprises an actuator for axially moving the objective lens in order to focus the radiation beam on the desired information layer.
It should be noted that when the objective lens is moved to switch from one information layer to the other a certain amount of spherical aberration (SA) is introduced. However, the numerical aperture of the radiation beam is relatively low resulting in an amount of spherical aberration within the tolerances of the optical scanning device, so that no spherical aberration compensation is needed. Actually, the amount of aberration is proportional to the fourth power of the numerical aperture of the beam, which means that a slight increase in the numerical aperture leads to a large increase in the amount of spherical aberration.
At present, the trend is to increase the numerical aperture of the radiation beam in order to reduce the size of the focus spot on the selected information layer and thus increase the storage capacity of data of the information layer. Actually, the diameter of the focus spot is inversely proportional to the numerical aperture. As a consequence, the amount of aberration introduced when switching from one information layer to another is such increased that it needs to be compensated.
It is worth noting that this need for SA compensation also applies to information carrier having only one information layer as in CD/DVD/BD compatible optical scanning devices where changing from one mode to another introduces undesired spherical aberration.
A prior art optical scanning device capable of compensating for SA aberration is shown in FIG. 1. Several mechanisms of SA compensation are known from literature:                Spherical aberration generating liquid crystal (LC) cell. Before the radiation beam is being focused on the disc 10 by the objective lens 6, spherical aberration is added to the wavefront compensating for the amount of spherical aberration due to the changing cover layer thickness. This can be done by locally changing the optical path in accordance with the change of information layer, for example by means of switching a liquid crystal in a LC cell 13.        
The drawbacks of this solution for SA compensation are: it is difficult to make for small pupil radii and intolerant to pupil displacement during radial tracking due to coma effect. As a result, the LC cell should be attached to the tracking actuator which is difficult to carry out for small form factor optical drives, because the weight on the tracking actuator would deteriorate the system performance.                Variable object conjugate distance. This method of SA compensation consists of generating spherical aberration inside the objective lens 6 itself. This can be achieved by changing the conjugate of the objective by adjusting the position of the collimator 4.        
The drawback of this solution is that for regular light path geometries with collimator having focal lengths of several millimeters it results in a rather large collimator displacement, typically several millimeters in high numerical aperture light path, which again is difficult to carry out in a small form factor optical drive.